My furnace will run for an hour or two, cycling 3 times each hour, then shuts down. I have to turn the temperature setting to "off" and then back up to 70°F, and it comes back on. I've replaced the thermostat and air filter, cleaned the flame sensor, cleaned the burners, checked all sensors with a meter (all have continuity). It's a Lennox G20Q3E-75-6, fairly old, not sure when it was installed.
Any ideas?